Generally, transmitters and receivers in radio equipment utilize the same antenna. Accordingly, the transmission network controls both the transmission signal and the receiving signal. By so doing, the signal from the antenna is directed to the receiver without any substantial interference from the transmitter, and the signal from the transmitter is transmitted to the antenna without interference from the receiver.
Ceramic filters are typically employed in radio equipment to reduce substantial interference. In particular, duplex ceramic filters including two individual band-pass filters are employed. One filter connects the receiving branch and has a center frequency and bandwidth corresponding to the receiving band. The other filter connects the transmission branch and has a center frequency and bandwidth corresponding to the transmission band.
One technique for tuning ceramic filters is to remove a conductive plating or ceramic dielectric material at the location of its top, patterned surface. This technique allows capacitive coupling between resonators. Alternatively, tuning may be accomplished by removing conductive material at the opposite, short-circuit surface of the filter.
During operation, a ground plane is required underneath the top, patterned surface is and attached to the side walls of the filter. Measurement of the ceramic filter can be accomplished with a mechanical fixture that connects the I/O ports on the ceramic filter to RF ports attached to measurement equipment. However, the ground plane blocks access to the top pattern during measurement. This prevents simultaneous measurement and probing of the coupling locations between individual resonators in the filter.